The present disclosure relates to electronic control of an electron beam. More particularly, circuits and methods for electron-beam energy control are disclosed.
Researchers have continually attempted to increase the storage density and reduce the cost of data storage devices such as magnetic hard-drives, optical drives, and dynamic random access memory (DRAM). It has, however, become increasingly difficult to increase storage density due to fundamental limits such as the paramagnetic limit, below which magnetically polarized materials are unstable at operating temperatures.
Several approaches have been used to increase storage density of data storage devices. One approach is based on scanned probe microscopy (SPM) technology. In such an approach, a probe is positioned extremely close to a storage medium. An example is atomic force microscopy (AFM) in which a probe is placed in physical contact with the storage medium. A second approach uses scanning tunneling microscopy (STM). In this approach, a probe is placed within a few nanometers of a surface of the storage medium to ensure that the probe is within a tunneling range of the medium. Although some success has been achieved using these approaches, it is difficult to economically manufacture data storage devices with probes that contact or that are in close proximity to the storage medium as these data storage devices require adequate protection schemes to prevent damage to the probe and/or the surface of the medium. Moreover, in STM, the distance from the probe to the medium must be precisely controlled. As known by persons having ordinary skill in the art, such control at the picometer scale is difficult to achieve.
In view of the difficulties associated with SPM, other researchers have developed methods that eliminate the need for extremely close proximity between the probe and the data storage medium. One such technique is based on near-field scanning optical microscopy (NSOM). Although NSOM avoids the precise control problem inherent with SPM, the NSOM technique has limited lateral resolution and bandwidth and consequently has limited practical applicability. Other techniques have been developed based on non-contact SFM, but these techniques typically suffer from poor resolution and poor signal to noise ratios.
Even where increased storage density can be achieved, hurdles to effective implementation exist. One such hurdle is the time required to access data stored on the storage device. Specifically, the utility of the storage device is limited if a relatively long time is required to retrieve the stored data. These data processing delays become more important with each increase in microprocessor clock speeds. Consequently, in addition to high storage density, there must be a mechanism for quickly accessing stored data.
Recently, semiconductor-based electron sources have been developed that can be used in storage devices and which may avoid the difficulties noted above. An example of such a data storage device is described in U.S. Pat. No. 5,557,596. The storage device described in the ""596 patent includes multiple electron sources having electron emission surfaces that face a storage medium. During write operations, the electron sources bombard the storage medium with relatively high-energy electron beams. During read operations, the electron sources bombard the storage medium with relatively low-energy electron beams. Such a device provides advantageous results. For instance, the size of storage bits in such devices may be reduced by decreasing the electron beam diameter, thereby increasing storage density and capacity and decreasing storage costs.
One type of electron source described in the ""596 patent is the xe2x80x9cSpindtxe2x80x9d emitter. As described in the ""596 patent, such an emitter has a cone shape with a tip from which electron beams can be emitted. Typically, the tip is made as sharp as possible to reduce operating voltage requirements and to achieve a highly focused electron beam. Unfortunately, utilization of Spindt emitters creates other problems. First, the fabrication of sharp emitter tips is difficult and expensive. In addition, focusing the electron beam from a Spindt tip in a temporally and spatially stable manner is difficult. Furthermore, the electron optics for focusing the emitted beams is complicated. Moreover, Spindt emitters do not operate well in poor vacuums. These problems become particularly important as the electron beam diameter is reduced below 100 nanometers.
Accordingly, alternative electron sources and focusing mechanisms are presently under development. Regardless of the type of electron source selected for the storage device, however, it is important to control the energy within the electron-beam directed at the data storage media.
Electron-beam control in ultra-high density storage devices such as that described in the ""596 patent presents a number of problems. First, mechanisms and or methods for directly measuring the electron-beam intensity incident at the surface of the data storage media would adversely effect the bit storage density and would add significantly to the cost of the storage device. Second, conventional current control techniques, when applied to electron emitters, causes the emitter voltage to vary over time. Consequently, controlling the electron beam by varying the emitter voltage results in an undesirable change in the overall potential between the surface of the storage medium and the emitter, thus changing the electron beam current in addition to the voltage potential inducing the beam. Third, a current control technique based on the assumption that the electron-beam current is a fixed fraction of the total current is susceptible to undesired variation in electron-beam power when the emitter efficiency is very low (e.g., in the range of 1% to 10%). Stated in another way, small variations in emitter efficiency would result in relatively large variation in the electron-beam current incident at the storage medium.
From the foregoing, it can be appreciated that it would be desirable to have a circuit and method for controlling the energy in an electron-beam generated by electron emitters that avoids one or more of the problems identified above.
Briefly described, in architecture, an electron-beam controller (EBC) capable of controlling the electron-beam power incident at the surface of a data storage medium can be realized with an emitter, an extractor, a current mirror, and an input current having a magnitude responsive to the desired electron beam current. For low-efficiency emitters (e.g., an emitter configured in such a manner that a significant portion of the total emitter current is sourced by an extractor) an EBC can be realized with the low-efficiency emitter coupled to a first current mirror, an extractor coupled to a second current mirror, and an input current having a magnitude responsive to the desired electron beam current, wherein the emitter and extractor currents are sensed to determine the actual beam current and the first current mirror adjusts the extractor voltage in response to the relationship between the desired electron beam current and the sensed electron beam current.
Other embodiments of the EBC may be realized in methods for controlling the power intensity of an electron-beam over time. A preferred method includes the steps of: (1) providing an emitter at a first voltage, (2) providing a target at a second voltage, (3) introducing an extractor at a controllable third voltage, (4) estimating the actual electron beam energy by sensing the emitter current; and (5) adjusting the third voltage in response to the sensed emitter current.